Intracellular potentials and sodium activity, (Na ion) i, will be measured in perfused proximal convoluted and proximal straight tubular segments of the rabbit kidney in vitro. Sodium selective liquid ion-exchanger microelectrodes combined with conventional microelectrodes will be used to measure (Na ion) i and the basal-lateral membrane potential, Vbl. The major objectives are to: 1) measure (Na ion) i in each tubular segment; 2) define the role of the luminal and basal-lateral membrane borders in determining (Na ion) i; 3) relate (Na ion) i to net sodium transport in each tubular segment; and 4) compare the mechanisms responsible for net sodium transport in the proximal convoluted tubule with those in the proximal straight tubule.